The invention relates to a vertical extending liquid/liquid contacting column. Such columns are for example used in the solvent extraction of aromatic hydrocarbons from a mixed hydrocarbon feedstock.
U.S. Pat. No. 4,336,106 describes a vertical column for the solvent extraction of aromatic hydrocarbons from a mixed hydrocarbon feedstock. The extractor column described in this publication is a vertical cylindrical column containing axially spaced sieve trays, which trays are provided with upcomers.
A disadvantage of the column of U.S. Pat. No. 433,106 is of a low efficiency and capacity.
The object of the present invention is to provide a column which, when used in an extraction operation, provides an improved efficiency and capacity.
This object is achieved with the following column.
A vertical extending liquid/liquid contacting column, which column will, when in use, contain a dispersed and a continuous liquid phase, the column having a first liquid feed inlet in the top, a first liquid outlet in the top, a second liquid feed inlet in the bottom, a second liquid outlet in the bottom and a plurality of internal trays axially spaced from each other in the column, each tray provided with a plurality of perforations for the passage of the dispersed phase and more than one downcomer or upcomer for the transport of the continuous phase, wherein the downcomer or upcomer extends respectively below or above the tray and wherein the walls of the downcomer or upcomer are inclined towards each other in the flow direction of the continuous phase.
Applicants have observed an improved capacity when the column according to the invention is used. Without wishing to be bound to the following theory, applicants believe that the improvement in capacity when compared to the capacity of a column as described in U.S. Pat. No. 4,336,106 is due to the short flow path length of the continuous phase on a tray in the column according to the invention. This effect will become even greater when columns having a wide diameter are used, such as wider than 2 m. The diameter is suitably smaller than 15 m.
The column may be provided with either upcomers or downcomers. A column will be provided with downcomers when the continuous phase in the column during contacting is the liquid having the higher density. When the continuous phase is the liquid having the lower density and consequently will move upwards in the column the column will be provided with upcomers. An example of a widely used liquid-liquid system which will require the use of upcomers is the sulfolane-hydrocarbon system in a process to remove light aromatics from naphtha streams boiling between 15 and 400xc2x0 C.
In the following description a tray will be described being provided with downcomers. Because the design of trays having downcomers is the same as for trays having upcomers, except that they will be mirrored in the horizontal plane of the tray, the preferred embodiments described below are also applicable to trays having upcomers.
The downcomer opening at tray level may be any shape, for example square, circular or rectangular. Preferably more than one of such downcomers are present in one tray. Preferably the downcomer opening has a rectangular horizontal cross section having a length greater than its width. A rectangular cross section also means a downcomer opening which smaller end side nearest to the circumferential of the tray runs along this circumferential, resulting that these smaller end sides are not arranged parallel relative to each other. This design of the downcomer opening ensures a maximum opening area on the tray. Preferably a downcomer opening is used having parallel smaller end sides. These downcomers are also referred to as boxed downcomers.
The tray layout may be such that the downcomer openings are evenly spaced in the area provided with the perforations. For example rectangular downcomers can be arranged in a row parallel to each other. At both ends of such a row a segmental downcomer may be present. This segmental downcomer will have a downcomer opening which at one end runs parallel to the rectangular downcomers and at the other end follows the circumferential of the tray.
Preferably rectangular downcomers are arranged on the tray in a so-called staggered arrangement. In this arrangement the tray is divided along a horizontal diametrical line in two tray sections, each tray section provided with more than one parallel arranged downcomer positioned perpendicular to the diametrical line. These downcomers extend from the wall, or near to the wall, to the diametrical line of the tray, such that the ends of the downcomers in one tray section meet the diametrical line alternating with the ends of the downcomers in the opposite tray section. Preferably 2-6 rectangular shaped parallel downcomers are present in one tray section. Additionally one or more segmental downcomers may be present on a tray section where the diametrical line meets the column wall. Such a segmental downcomer will have an opening which runs parallel to the longitudinal side of the rectangular downcomers, along the circumferential of the tray and along part of the diametrical line.
When the staggered trays are placed in a column, two consecutive trays will be mirror images of each other with the diametrical line as mirror. This ensures that that the lower end of the downcomer is positioned above the perforations of the tray below. The staggered tray layout is preferred because it prevents vertical by-passing of the continuous phase or said otherwise, it increases the horizontal mixing of the continuous and dispersed phase. A staggered tray is preferably constructed such that the downcomer is supported at one smaller end by a central supporting beam placed along the diametrical line and at the other end by a circumferential beam running along the inner wall of the column.
Preferably the lower or continuous discharging end of the downcomers of one tray are spaced away from the upper end or continuous phase receiving end of the downcomers of the tray just below said tray. This distance is referred to as flow path length and for this invention is defined by the shortest horizontal distance between the center of the continuous phase discharging downcomer and the center of the continuous phase receiving downcomer. Preferably this flow path length is between 0.05 and 0.5 m. A shorter flow path length would result in a less efficient contacting. A greater length would result in more turbulence of the continuous phase above a tray, which in turn would hinder the transport of the dispersed phase, thereby reducing the capacity.
The liquid discharge end of a downcomer is preferably placed at some distance above the tray just below. Preferably the liquid discharge end extends between 30 and 80% of the tray distance below the tray. Preferably the receiving end of the downcomer extends between 0 and 20% of the tray distance above a tray. The tray distance is the distance between two consecutive trays.
The perforations in the tray are preferably simple openings which easily allow the lighter liquid to pass the tray in an upward direction. Preferred openings are sieve openings having a preferred diameter of between 0.004 and 0.025 m. The total area of the perforations on the tray relative to the cross sectional area of the column will depend on the choice of liquids to be contacted and flow rates in the column. Suitably this area is between 2 and 20% of the cross sectional area of the column.
The downcomer has walls which are inclined towards each other in the downward direction. These walls extend partly below and optionally partly above the tray. For construction reasons it is advantageous that the walls of the downcomer extend somewhat above the tray. The slope of these walls is suitably between 1 and 45 degrees relative to the vertical axis of the column. The lower end is preferably provided with a cover, which cover is provided with, preferably downward directed, openings. The dimensions of these openings are preferably so chosen that the continuous phase exiting the openings is accelerated such that its velocity is more than double the velocity of the rising dispersed phase. The downcomer area as measured on the tray level will depend on the specific liquids to be contacted. Preferably this area is between 2 and 20% of the cross sectional area of the column.
The column is preferably provided with between 2 and 100 trays as described above. The number of trays will depend on the specific liquid liquid system to be contacted and the extraction depth required. The column is furthermore preferably provided with a coalescer at the top of the column positioned between the trays and the top liquid outlet. This coalescer improves the phase separation of the dispersed and continuous phase after contacting. Preferably this coalescer has the form of structured packing and is made of a material which has a higher affinity to the dispersed phase than to the continuous phase.